Gaston et al. 1998

 Gaston et al. 1998.

Rapoport’s rule: time for an epitaph?. 1998. Gaston, K. J., Blackburn, T. M., & Spicer, J. I. TREE 13: 70-47.

Blog author: Laura Segura-Hernandez
Paper Authors: Kevin J. Gaston, Tim M. Blackburn and John I. Spicer.

Kevin J. Gaston:

·     PhD from University of York

·     Professor of Biodiversity and Conservation at University of Exeter.

·      Research interests: “I conduct basic, strategic and applied research in ecology. This is presently centred around three main issues: (1)Common ecology – the study of common species, the determinants of commonness and its consequences. (2)Nighttime ecology – the study of the abundance, distribution and interactions of species during the night (including the consequences of anthropogenic pressures such as artificial nighttime lighting). (3) Personalised ecology – the study of the direct interactions between individual people and nature, their causes and consequences.”

Tim J. Gaston:

·     PhD completed in 1991

·     Professor of Invasion Biology at University College London

·     Research interests: “My research primarily addresses questions relating to human-mediated biological invasions. Invasions are now so pervasive that alien species are a central element of current global environmental change, and a major drain on economic resources, giving a strong incentive to understand the process that leads to a species becoming invasive.” 

John I. Spicer:

·     PhD from Glasgow University.

·     Professor of Marine Zoology at University of Plymouth.

·     Research interests: I couldn’t find a summary of his interests, but in his profile in Research Gate he lists these topics as his main skills and expertise: Environment, Biodiversity, Ecology, Climate Change, Marine Ecology, Physiology.

Summary/Main points

1. Main Question: 

·   background: In 1989, Stevens published a paper in which he proposes that the observation made by Eduardo H. Rapoport (that the geographic range of species declines in relation to the latitude) is a rule. This attracted attention, especially of those seeking to understand the patterns of species richness in the latitudinal gradient. However, previous rules proposed as general trends in species richness have already been questioned about their alleged generality. And as different studies present contradictory results in regards of Rapoport’s rule, this might not be exception. 

·   main question:Is the proposed Rapoport’s rule indeed a rule? What does the information compiled so far tell us about its universal applicability? 

2. Methods:

·   data: results compiled out of approximately 22 different studies (Table 1).

·   Methods:  They organize the results of the 22 studies in one single table, in order to compare the methods used, taxa analyzed, results and conclusions. That way, they assessed how does studies could support, or not, Rapoport’s rule.

3. Results: 

·   Limitations in the methodology applied to test Rapoport’s rule:

·   Central tendency: the way in which Stevens measured range size (average range size of species that occurred in different bands of latitude) is not a good way to measure the central tendency of range size. And the central tendency measure used has been proven to have an effect in analyses of latitudinal patterns 

·   Spatial non-independence: Stevens method doesn’t have independent measures of size range, as the averages of bands that are close together will probably include the same species. 

·   Phylogenetic non-independence: species might be related to each other, therefore, they don’t represent an independent data point and that might bias the analyses.

·   Boundedness: hard limits, such as physical barriers, might be the drivers that constraint the size of latitudinal range.

·   Evidence to prove Rapoport’s rule: 

·   The idea that all species showed a decline in their range as their distribution got closer to the Equator was proven equivocal. There is not a general pattern in the taxa analyzed.

·   There is a local pattern though. It can be observed during the Palaearctic and Nearctic, in latitudes above 40-50°.

·   Mechanisms that might explain the pattern when it exists:

·    Land area: Many analyses have been done in the New World, therefore, the pattern observed might be due to the shape of the continent and the available land area. However, evidence showed in mammals, birds and in the Palearctic dismisses that this is the cause for the patterns.

·   Climatic variability:Stevens argued that the climatic variation would be the driver for determining the distribution of species. This has been supported by several studies. However, aa study done in Palearctic mammals, 3 different ways of measuring seasonality were analyzed. 2 of them (daily temperature range and variance of annual precipitation) showed a negative relationship with range size, while the other one (annual temperature range) showed a positive relationship and this one explains more of the variance. However, this doesn’t explain why the pattern is shown or not shown in other taxa.   

·   Differential extinction:   species in higher latitudes would have larger ranges because those species that were not able to endure the conditions are extinct now and the remaining species occupied those spaces. This might not be totally false due to the faact that the pattern is shown in the Northern latitudes. However, this might not be true in other ranges.

·   Competition:lower species richness leads to less competitive interactions. But there is not evidence showing that competition is a limitation to geographic distribution of species.

·   Biogeographical boundaries: the extension of the biomes might limit the distribution, as moving within the same biome is easier than moving between geographic provinces. Also, biogeographical provinces are larger at higher latitudes. This seems to explain the patterns observed in the distribution of birds and molluscs. But further research must be done.

4. Conclusions/Inference

·   The support to call Rapoport’s rule, a rule, it not strong. However, there is evidence to suggest that this trend exists and therefor it can be named the “Rapoport’s effect”. This might be a local effect present in the northern latitudes. More studies need to be done in the southern latitudes to confirm if the effect is also present there.

·   There is not a single possible mechanism that might explain this pattern.  Studies assessing not just the pattern, but its causes, are needed. 

5. Questions/comments: 

1.  I usually don’t enjoy reading reviews, but I think this one was concise and clear. The authors pointed clearly why Rapoport’s rule is not a rule, but I liked that they didn’t completely disregarded it by changing it to an “effect”. Do you think it is correct to change it to “effect”?.

2.  I enjoyed how they pointed how, even the papers where the effect was not found, this topic helped to pay attention to different explanations to the patterns that were first pointed by Rapoport.

3.  I found it interesting how the person that first noted this pattern is from South America, and now, one of the main things lacking is the information from those lower latitudes.